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Revisiting Old Stories About Little Airways

Everyone who gets taught a bit about the paediatric airway hears a few classic stories. Well the thing about the classics is sometimes they need an update. Andrew Weatherall revisits kiddy airway stories now that we know more stuff.

There are some things you’re taught from a very young age to believe in. Then it turns out it’s just plain wrong. Santa Claus. The Tooth Fairy. The Public Holiday Numbat. (Well, the last one might be specific to my upbringing.)

And in medicine there are plenty of examples those too. Oxygen is always good. You can’t manage trauma without a cervical collar. Then of course there’s pretty much everything about the paediatric airway. As if managing kids didn’t come with challenges anyway, we all get to work with information that is just plain wrong.

And there’s no mistaking that clinicians find paediatric airways difficult. The staff from Royal Children’s Hospital Melbourne have recently published a sizeable prospective study of emergency department intubations. This is from a big, clinically excellent tertiary kids’ hospital receiving 82000 patients in their ED every year. In 71 intubations across a year (only 71!), 39% had adverse events (most commonly hypotension in 21% and desaturation in 14%) and the first pass success rate was 78% (only 49% had a first pass intubation with no complications).

Now lots of things will contribute to those figures (not least of which is paediatric emergency practitioners obviously don’t get the airway management experience of those wafting around the vapours). But at least part of our pondering has to be making sure we understand what we’re dealing with.

It points out that some of the classic teaching on the paediatric airway come out of a 1951 report by a Dr Eckenhoff. This includes the issues of the position of the larynx, the shape of the epiglottis and the funnel-shaped airway. Actually, to really trace the story, you have to start a little earlier.

Stepping Back

It’s 1897. Waistcoats aren’t ironic yet. Pipes aren’t an affectation they’re an expectation. Jack the Ripper is part of shared memory, not fevered historical narratives. And Bayeux was making casts of the airways of dead children. 15 casts actually in kids aged 4 months to 14 years.

Taking measurements of the circumference of the airway at the glottis, cricoid level and trachea, the cricoid ring was noted to be narrower than other parts of the airway (the topic of the shape of the airway wasn’t mentioned). This is the work that led to the idea that kids under the age of 8 had a conical larynx, with the cricoid ring as the narrowest point.

Consider for a second the qualities of plaster poured into a distensible tube. Wait, it’s not entirely distensible because the cricoid can’t distend. Is it maybe possible that the plaster may have distorted the anatomy? I’ll leave that with you for a bit.

This suggestion of the conical airway made its way into Eckenhoff’s later paper (though with a specific note that cadavers may not represent the living accurately). There were also some descriptive points raised:

The larynx moves down from the C3-4 level in the neonate to C4-5 in the adult (I’ve always been under the impression this move is brought about both by the need to phonate properly for speech and the loss of the need to breathe and breastfeed at the same time, but this point doesn’t feature in airway descriptions and I’m happy to be corrected).

A stiffer and more “U” or “V”-shaped epiglottis with an angle to the anterior pharyngeal wall of around 45 0 rather than lying close to the base of the tongue.

A case report of a 2 year old with airway complications thought to be related to an inappropriately sized tube, feeding the idea of uncuffed endotracheal tubes in kids under the age of 8.

All these points that form part of so much teaching lead to another question – would such a descriptive effort get a run in modern publishing?

Newer Tools Means Better Understanding

The answer of course is probably not. Of course you can only use what you have and it’s absurd to judge Eckenhoff (or Bayeux) for their accuracy against modern modalities. All we can do is revisit our thinking when new information becomes available.

We now have the significant advantage of radiological techniques (CT or MRI) and bronchoscopy to evaluate airways in children who aren’t dead. Again the Tobias article goes into more details but there are some key things to take from this modern literature:

In spontaneously breathing and muscle relaxed patients, the cricoid was not the narrowest part of the airway. That honour belongs to the vocal cords.

There is no change in the ratios of the cross sections over age – the cricoid doesn’t start relatively smaller and enlarge by the time you hit 8.

The cross-section looks like an ellipse (there’s more distance between the anterior and posterior bits than the two side bits).

What should we do then?

Well for starters we should probably settle the tube choice thing. This is just more support for the argument to use a cuffed tube. For starters, the old “leak” test seems pretty dubious when you could be snug against the lateral walls but still leaking around the anterior or posterior areas. And I’m guessing no one has had their “leak accuracy assessment” externally audited.

It makes more sense to use an appropriately sized cuffed tube with the cuff pressure kept < 20 cm H2O. There’s now fairly convincing evidence that appropriately used cuffed tubes don’t cause big issues in recovery.

The key though is “appropriately used”. A paper coming out in Anesthesia and Analgesia writes headlines suggesting vastly higher rates in neonates having a Microcuff endotracheal tube compared to an uncuffed tracheal tube. Except maybe that’s the equivalent of academic clickbait.

Take a closer look and in the 29 patients with a cuffed tube (vs 295 with an uncuffed tube), 26 of them had a tube larger than recommended. Plus the other risk factors for stridor were post-conceptual age at birth being lower (29.9 weeks vs 33.0), birth weight being almost 25% less (1.56 vs 2.02 kg) and requiring reintubation or being intubated for longer (20 days vs 3). So maybe the conclusion should be “choosing endotracheal tubes that are bigger than the anatomy allows, needing it in for a long time and requiring repeat snorkel application is associated with stridor”. There, fixed it.

So if you choose the tube wisely life is easier. Better ventilation, better monitoring, less flows and gentler tube material in contact with the mucosal wall. Makes sense.

The other thing you can’t do is ignore the cricoid, even if it’s not the smallest part of the airway. It is still an unyielding bit of the anatomy and anyone can turn a high volume-low pressure cuff into a high volume-high pressure cuff – the difference is a couple of mL. And swelling in an airway that starts with a much smaller cross-sectional airway still means less margin for flow obstruction.

So choose the right tube, use it safely and you can get on with things.

While We’re At It, Let’s Forget One Blade to Rule Them All

Seeing as we’re talking about things that aren’t things, you may have also come across the idea that you should use a straight blade for the smaller kids (say, kids under 2). I’ve mentioned elsewhere that I think this is baloney but here’s a little bit of evidence.

Varghese and Kundu have published something on exactly this issue. 120 kids aged from 1-24 months had laryngoscopy (once anaesthetised and given muscle relaxation) with either a Miller or Macintosh blade, and then crossed over to the other type of blade. (Note they used both with the tip in the vallecula.)

The findings? The views were pretty much the same. The rates of difficulty were about the same. In fact, it’s a pretty beige set of numbers where being beige is actually as cool as things could be.

Some where the view wasn’t so great with a Macintosh had a better view with the Miller blade. Some went in reverse. The message though is a pretty resounding “same, same”.

So there’s just some truths that needed revisiting. There are no funnel-shaped airways. The airway isn’t round. There’s not one correct blade for the under 2s.